Gibberellins (GA) are a plant hormone family of tetracyclic diterpenoids, compounds essential for diverse developmental processes in plants, including growth, flowering, and fructification (Non-patent Document 1). In particular, by regulating the activity of GA, one can control the growth and differentiation of various plants. For example, crop yield varies greatly depending on the type of plant, and therefore can be increased by improving the plant type via regulation of GA signaling. However, to date, it is unknown of how plants perceive GA, how the GA signal is transmitted, and how plant growth is induced by GA regulation. Recently, the present inventors have isolated two types of GA-signaling mutants in rice. One is a constitutive GA response mutant, slender rice1 (slr1) (Non-patent Documents 2 and 3); the other is a GA-insensitive dwarf mutant, GA-insensitive dwarf2 (gid2) (Non-patent Document 4). The SLR1 gene encodes a protein predicted to be a transcription factor orthologous to Arabidopsis GAI (Non-patent Document 5) and RGA (Non-patent Document 6), wheat Rht, maize d8, and barley SLN1 (Non-patent Document 7). All these proteins, including and mentioned after Arabidopsis GAI above, are grouped into the DELLA subfamily of the GRAS family (Non-patent Document 8). The GID2 gene encodes a protein that is predicted to be the F-box subunit of SCF E3 ubiquitin ligase, which is orthologous to Arabidopsis SLY (Non-patent Document 9). Analyses of these mutants have predicted that the rice SLR1 protein, a GA-signaling repressor, is degraded via the SCFGID2-proteosome pathway that induces the actions downstream of GA (Non-patent Document 8). However, far less is known about the process of GA perception as compared to the process regulated by DELLA proteins. To date, there are several reports on GA-binding proteins studied by biochemical approaches (Non-patent Documents 10 and 11). However, as yet, no proteins have been identified that directly participate in GA perception.    Non-patent Document 1: Davies, P. J., Plant Hormones (Kluwer Academic, Dordrecht, Netherlands, 1995).    Non-patent Document 2: Ikeda, A. et al., Plant Cell. 13, 999-1010 (2001)    Non-patent Document 3: Itoh, H. et al., Plant Cell 14, 57-70 (2002)    Non-patent Document 4: Sasaki, A. et al., Science 299, 1896-1898 (2003)    Non-patent Document 5: Peng, J. et al., Genes Dev. 11, 3194-3205 (1997)    Non-patent Document 6: Silverstone, A. L. et al., Plant Cell 2, 155-169 (1998)    Non-patent Document 7: Gubler, F. et al., Plant Physiol. 129, 191-200 (2002)    Non-patent Document 8: Itoh, H. et al., Trends Plant Sci. 8, 492-497 (2003)    Non-patent Document 9: McGinnis, K. M. et al., Plant Cell. 15, 1120-1130 (2003)    Non-patent Document 10: Lovegrove, A. et al., Plant J. 15, 311-320 (1998)    Non-patent Document 11: Nakajima, M. et al., Biochem. Biophys. Res. Comm. 241, 782-786 (1997)